Car Mechanics (UK) - - Instant Expert -

Tim­ing belts rarely snap. More com­monly, the fi­bre teeth will shred, or else the belt sim­ply falls from its pul­leys. Nat­u­rally, leav­ing the belt past a rea­son­able re­place­ment in­ter­val can lead to ex­ces­sive de­te­ri­o­ra­tion of its struc­ture. Yet, as a tim­ing belt tends to not only drive the camshaft but also other items, such as the oil/ wa­ter/high-pres­sure fuel pumps, or the bal­ance shafts, any wear of those items, in­clud­ing within their bear­ings, will place the belt un­der ad­di­tional stress. The belt is likely to be routed around idler and ten­sioner pul­leys, with de­te­ri­o­rat­ing bear­ings on those com­po­nents af­fect­ing belt longevity also. Should the ten­sioner fail, for ex­am­ple, the slack belt can sim­ply come away from its pul­leys.

As belt life is linked so di­rectly to the health of those other com­po­nents, it is wise to not re­place the belt alone. For this rea­son, most tim­ing belts are sup­plied as part of a kit that should in­clude a ten­sioner and idler at the very least. How­ever, you should con­sider tak­ing the op­er­a­tion one step fur­ther, by re­new­ing other items. Wa­ter pumps, for ex­am­ple, tend to re­quire re­plac­ing at least ev­ery other belt change and this gives you the op­por­tu­nity to re­fresh the coolant, re­new­ing the cool­ing sys­tem’s anti-cor­ro­sive prop­er­ties that di­min­ish nat­u­rally with time.

Con­sider the con­di­tion of any parts that might fail and get tan­gled within the tim­ing belt. Aux­il­iary, or bal­ance shaft, belts can come off their pul­leys and be dragged into the tim­ing belt cover. Linked to this is a crank­shaft pul­ley, many of which in­cor­po­rate an in­ter­nal damper (see Tor­sional vi­bra­tion dampers – TVD).

As ex­ter­nally-mounted tim­ing belts op­er­ate ‘dry’, the pres­ence of oil, in par­tic­u­lar, will de­grade their struc­ture, so at­tend­ing to any coolant and oil leaks must be a pri­or­ity. Ob­vi­ously, no ro­tat­ing belt should be al­lowed to abrade against any­thing and a part that has been fit­ted cor­rectly should run freely – al­though miss­ing bolts on the cover can cause the belt to rub through the plas­tic cas­ing.

Belt life can also be short­ened by in­cor­rect fit­ting and care­less han­dling. Never crimp or twist a belt, be­cause you can break the in­ter­nal re­in­forc­ing fi­bres. Never con­tam­i­nate the belt with dirt or oil ei­ther – so keep your hands clean.

Apart from han­dling is­sues, get­ting the ten­sion wrong is the most com­mon mis­take. Misalign­ing and over­tight­en­ing the belt can be just as dam­ag­ing as not ten­sion­ing it suf­fi­ciently. A belt that it too tight might pro­mote a whin­ing sound with the en­gine run­ning; even­tu­ally, its teeth can strip. Overten­sion­ing also places an ad­di­tional strain on other com­po­nents, such as the high-pres­sure diesel pump bear­ings.

Get­ting the ten­sion right is not al­ways easy and meth­ods dif­fer be­tween en­gines. Some ten­sion­ers are spring-loaded. The one pic­tured here is not and re­quires a force of 30 new­tons to be ap­plied to the belt and its de­flec­tion checked as be­ing 7.5mm at a spe­cific point with the en­gine cold.

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